The present invention relates to integrated circuit (IC) packaging and, more particularly, to substrates used in IC packaging.
Surface-mount grid-array-type integrated circuit (IC) devices such as, for example, land grid array (LGA), ball grid array (BGA), and pin grid array (PGA) packages, electrically connect a die to a corresponding printed circuit board (PCB) through pad openings in their bottom surfaces. In contrast, some other surface-mount IC devices such as, small-outline IC (SOIC) or quad flat pack (QFP) packages—connect to a corresponding PCB through lead fingers emerging from the package sides.
The particular attachment mechanism between the pad openings of a grid-array-type IC device and a PCB may be, for example, (1) solder balls—as used for BGA devices, (2) pins on the IC device, which fit in a corresponding socket on the PCB—as used for PGA devices, or (3) pins on a corresponding socket on the PCB—as used for LGA devices. The pad openings are at the bottom of a PCB-like substrate of the packaged device. The package substrate may comprise materials similar to those of a typical PCB and may be similarly fabricated. Note that a package substrate is not the same as an IC die substrate, which is typically semiconductor bulk material, such as crystalline silicon. As used herein, unless otherwise indicated, the term “substrate” refers to a package substrate and not to an IC die substrate.
Each IC die of a grid-array-type package is typically mounted on the top side of the substrate and electrically connected to bond posts on the top side of the substrate. The die and top side of the substrate are then encapsulated in molding compound. Metallic vias connect the bond posts on the top side to corresponding pad connectors on the bottom side of the substrate. The pad connectors, in turn, electrically connect the substrate to the PCB. In a conventional new product introduction (NPI) process, a custom substrate is designed for a particular device based on the dimensions of the to-be-packaged die(s).